To rephrase the title of this post: How likely is RCP8.5? Answer: it depends who you ask. Last year’s Fourth National Climate Assessment does not assign a “formal likelihood” to any of the RCP scenarios. However, Scientific American just ran a piece “EPA Head Targets “Worst-Case” Climate Scenarios” claiming “experts in climate modeling” estimate there’s a 10% chance RCP8.5 could be surpassed by the end of the century. The author does not include a link or reference to any report or peer-reviewed paper confirming the 10% probability. Upon further research I found Capellan-Perez et al (2016), who provided a 12% probability of reaching RCP8.5 by 2100. On the other hand, we have Ritchie and Dowlatabadi (2017), who say the probability of reaching RCP8.5 by 2100 is “virtually zero”.

Why the difference in expert opinion about the likelihood of the RCP8.5 worst-case scenario? Because climate models generate different futures depending on their assumptions and inputs. For instance, the authors claiming a 12% probability of RCP8.5 assume “fossil fuel resource availability is a key driver of emission pathways” and rely on relatively high estimates of coal reserves. According to this scenario, humans won’t transition to renewables until fossil fuels start running out and by then it will be too late to avoid catastrophic climate change.

The assumption of a coal-dominated near-future is not specific to Capellan-Perez - it’s been standard in RCP8.5 climate models for years. For instance, check out this chart from an influential 2011 paper, in which projected RCP8.5 coal consumption just about equals all other sources of energy use combined:

Are current trends pointing to the long-term dominance of coal? Not according to the following:

My take-away: to the extent that climate change worst-case scenarios are based on inaccurate assumptions about coal reserves, they are not plausible. Show me an RCP8.5 without that assumption and I may change my mind.